A training sequence is a series of apriori known symbols which are transmitted at predetermined times from signal transmitter to signal receiver. Such sequences have long been used to adjust the operation of signal dispersion compensation apparatus in the receiver, such as equalizers, echo cancellers and the like.
Signal dispersion, such as echoes or signal "ghosts," is an inherent problem in communications systems and the severity of the problem can vary with the system application. For example, signal ghosts or echoes at levels which are merely objectional to the viewer of a received conventional television signal can render a high definition television (HDTV) signal unintelligible. Accordingly, compensation for signal dispersion is required in this and in many other communications systems. Moreover, while signal dispersion compensation apparatus, such as equalizers and cancellers, provide satisfactory compensation, knowledge of the signal dispersion characteristics, e.g., amplitude, delay and phase, is useful for the adjustment of such apparatus during system start-up and operation.
Prior art techniques exist which can determine the characteristics of signal dispersion. Such techniques typically transmit specific signals, such as square wave pulses or training sequences, which are detected and analyzed in the receiver. These techniques provide satisfactory estimates of the signal dispersion characteristics in applications wherein the dispersion is large in amplitude and short in duration. However, the prior art techniques provide inaccurate results when the signal dispersion is small in amplitude, particularly in the presence of channel noise, jitter or similar impairments, and provide ambiguous results when the signal dispersion is long in duration. These shortcomings have hindered development of communications systems requiring ever-more precise signal dispersion compensation and more precise determination of the signal dispersion characteristics. Therefore, a technique which provides greater accuracy in the determination of small-amplitude and long-duration signal dispersion would be desirable.